Cycles Viewport Render not updating while Texture Painting
My answer to this question explains most of the issue.

The key points to make texture painting work while Cycles is set as the render engine are -


The 3DView shading mode should be set to Texture
The object needs to have a UV Map
The object needs to have a material that has an image node set to the desired image


When the material has multiple image nodes the selected image node defines the active image being drawn to. The image node does not need to be connected to any other nodes.

The UV Map selected in the UV Map list is the one being used to display the image selected. Any UV Maps linked into the image node are ignored with the 3DView set to Texture shading.

While you can switch the render engine to Blender Render while you do texture painting, the only real difference is that Cycles chooses the image from an image node were Blender Render links the image in the UV Image Editor to a UV Map while in edit mode.